Multiple recombinant gene transfer vectors based on different types of viruses have been developed and tested in clinical trials in recent years. Gene transfer vectors based on adeno-associated virus (AAV), i.e., AAV vectors, have become favored vectors because of characteristics such as an ability to transduce different types of dividing and non-dividing cells of different tissues and the ability to establish stable, long-term transgene expression. While vectors based on other viruses, such as adenoviruses and retroviruses may posses certain desirable characteristics, the use of other vectors has been associated with toxicity or some human diseases. These side effects have not been detected with gene transfer vectors based on AAV (Manno et al., Nature Medicine, 12(3):342 (2006)). Additionally, the technology to produce and purify AAV vectors without undue effort has been developed.
At least 11 AAV serotypes have been identified, cloned, sequenced, and converted into vectors, and at least 100 new AAV variants have been isolated from non-primates, primates and humans. However, the majority of preclinical data to date that involves AAV vectors has been generated with vectors that are based on the human AAV-2 serotype, which is considered the AAV prototype.
There are several disadvantages to the currently used AAV-2 vectors. For example, a number of clinically relevant cell types and tissues are not efficiently transduced with these vectors. Also, a large percentage of the human population is immune to AAV-2 due to prior exposure to wildtype AAV-2 virus. It has been estimated that up to 96% of all humans are seropositive for AAV-2, and up to 67% of the seropositive individuals carry neutralizing anti-AAV-2 antibodies which could eliminate or greatly reduce transduction by AAV-2 vectors. Moreover, AAV-2 has been reported to cause a cell mediated immune response in patients when given systemically (Manno et al., Nature Medicine, 12(3):342 (2006)).
Methods of overcoming the limitations of AAV-2 vectors have been proposed. For example, randomly mutagenizing the nucleotide sequence encoding the AAV-2 capsid by error-prone PCR has been proposed as a method of generating AAV-2 mutants that are able to escape the neutralizing antibodies that affect wildtype AAV-2. However, it is expected that it will be difficult to generate significantly improved AAV-2 variants with single random point mutations, as the naturally occurring serotypes have only about 85% homology at the most in the capsid nucleotide sequence.
Methods of using a mixture of AAV serotype constructs for AAV vectors have also been developed. The resulting chimeric vectors possess capsid proteins from different serotypes, and ideally, thus have properties of the different serotypes used. However, the ratio of the different capsid proteins is different from vector to vector and cannot be consistently controlled or reproduced (due to lack of genetic templates), which is unacceptable for clinical use and not satisfactory for experimental use.
A third approach at modifying the AAV-2 capsid are peptide insertion libraries, in which randomized oligonucleotides encoding up to 7 amino acids are incorporated into a defined location within the AAV-2 capsid. The display of these peptides on the AAV-2 capsid surface can then be exploited to re-target the particles to cells or tissues that are otherwise refractory to infection with the wildtype AAV-2 virus. However, because knowledge of the atomic capsid structure is a prerequisite for this type of AAV modification, this method is currently restricted to AAV serotype 2. Moreover, peptide insertion libraries typically cannot address the issues of AAV particle immunogenicity or transduction efficiency.
Thus, there remains a need for new AAV vectors and a method of generating new AAV vectors. In particular, there is a need for AAV based vectors that can be used efficiently with a variety of cell types and tissues and that do not react with a pre-existing anti-AAV human immunity that could neutralize or inactivate the vectors. There also remains a need for vectors that transduce different cell types in vivo and in vitro and that offer a more restricted biodistribution or a more promiscuous biodistribution, depending on what may be required. In particular, there remains a need for vectors capable of transducing a variety of cells types, such as hematopoietic stem cells or embryonic stem cells.
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